The rapid growth of subscribers in Global System for Mobile Communications (GSM) networks has led to need to increase system capacity. Voice Services Over Adaptive Multiuser Orthogonal Subchannels (VAMOS) is one technique for increasing system capacity in a GSM network. VAMOS allows two wireless terminals to be multiplexed into the same time slot on the same frequency. In the downlink, Adaptive Quadrature Phase Shift Keying (AQPSK) is used. The in-phase (I) and quadrature (Q) components of the complex baseband signal are treated as two sub-channels. The signal from one user is mapped to one sub-channel of an AQPSK modulated carrier and the signal for the other user is mapped to the other sub-channel of the AQPSK modulated carrier. Orthogonal training sequences are transmitted on each sub-channel and are used by the receivers in the wireless terminals to separate the two signals. More particularly, each wireless terminal uses its assigned training sequence to separate the desired signal transmitted on one sub-channel from the interfering signal transmitted on the other sub-channel.
VAMOS incorporates a sub-channel power control feature that allows the base station to divide the total downlink transmit power unequally between the sub-channels of the AQPSK modulated carrier. For example, the base station may allocate power to the sub-channels depending on the respective path losses to the paired wireless terminals, referred to hereinafter as a VAMOS pair. The sub-channel power control feature makes it possible to pair wireless terminals with different channel conditions to increase the VAMOS coverage area and to increase the number of possible VAMOS pairs. Thus, extra power can be allocated to one of the sub-channels at the expense of the paired sub-channel. When the power imbalance is large and favors the interfering signal, Successive Interference Cancellation (SIC) can be used to subtract the interfering signal from the received signal before demodulating the desired signal. However, the interference attributable to the interfering signal cannot be completely removed.
Parameters, such as Doppler shift, frequency offset, and timing error used to control the receiver are typically estimated from the desired signal. The residual interference after SIC in the case where the interfering signal is stronger can degrade the parameter estimation.